1. Field of the Invention
This invention relates to a control equipment for a resistance welding machine, and more particularly to a control equipment for a resistance welding machine for welding a material by the Joule's heat generated in the material by applying an electric power to the material.
2. Description of the Related Art
As a control equipment for a resistance welding machine, there is an equipment which uses a commercial power source as it stands as the welding current power source. There is also an equipment which uses a commercial power source by first converting it to a DC voltage and then converting the DC voltage to an AC voltage of the desired frequency by using an inverter.
FIG. 41 shows a case of using a commercial power source as it stands. In FIG. 41(a), the voltage of a commercial frequency AC power source 101 is controlled of the trigger phase by thyristors 102 connected in antiparallel. Thus a controlled variable voltage is applied to the primary winding of a transformer 103. A secondary winding of transformer 103 is connected to welding electrodes 105. Resistance welding is performed by passing current to a metal object to be welded which is sandwiched by electrodes 105.
The magnitude of the welding current is set as an effective value by a current setting unit 107. The primary current of transformer 103, detected via a current detector 104 is converted to an effective value by an effective value computing unit 108. Both these currents are compared by a current control unit 109, and a control signal is outputted to reduce their error. The trigger phase is then determined by a phase control unit 110 based on the control signal. When a conduction starting instruction is applied from a starting circuit 112, a conduction instruction is outputted for a specific time period which has been set in timer 113. A driving circuit 111 outputs a trigger pulse at the trigger phase outputted from phase control unit 110 to switch thyristors 102 ON. In this case, as shown in FIG. 41(b), trigger phases .alpha.1, .alpha.2 of the positive half-wave and the negative half-wave of 1 cycle of the power source voltage are controlled as a pair. Also, due to the leakage inductance of transformer 103 and the floating inductance 106 which is present in the cable between the secondary winding of transformer 103 and welding electrodes 105, the load viewed from the primary side of transformer 103 becomes a delayed power factor. Thus, welding current I flows, as shown by FIG. 41(b).
In this method, the magnitude of the welding current always changes. Thus, there is great fluctuation in the heat at the welding portion and therefore there are limits to the welding quality. Also, since control is performed every one cycle of the commercial frequency, there are limits to the control response. Thus, if welding is performed by the optimum welding current without an enough margin, inferior products will sometimes occur. Also, as the power factor of the load becomes low and the load becomes a single phase load, there is the problem of unbalance occurring in a 3-phase power source load.
The inverter is used in a control equipment as shown in FIG. 42 is used to solve these problems. In FIG. 42(a) a DC voltage is obtained from 3-phase commercial frequency AC power source 101 via a rectifier 114. The DC voltage, which has been smoothed by a capacitor 115, is pulse-width modulated (PWM) by inverter 116 and is converted to AC voltage V1 with a high frequency of 600-1000 Hz, which is applied to the primary winding of transformer 103. The AC voltage which is induced in the secondary winding of transformer 103 is full-wave rectified by diodes 117 and is converted to DC voltage V2. Thus, DC welding current I flows in the object to be welded which is sandwiched by welding electrodes 105.
The design is such that the welding current is determined by current setting unit 107. A primary current of transformer 103 detected via current transformer 104 is converted to a welding current by a simulation circuit 118. These currents are compared by current control unit 109, and a control signal is outputted to make their error zero. This control signal is compared with a high-frequency triangular wave outputted from a carrier generating unit 119 in a PWM control unit 120 and is outputted to driving unit 111 as a PWM control signal. The PWM control signal is outputted from driving unit 111 while the conduction instruction is inputted via starting circuit 112 and timer circuit 13. Thus, inverter 116 outputs the PWM controlled AC voltage V1 shown in FIG. 42(b). A low AC voltage of a waveform similar to this voltage V1 is induced in the secondary winding of transformer 103, and is converted to a full-wave rectified DC voltage V2 by diodes 117 having a large conduction capacity. Thus, a DC welding current I flows as shown in FIG. 42(b).
This method makes high-quality welding possible by solving the problems described above. Moreover, there is advantage of making the transformer small and of light weight by adopting high frequency.
However, in the prior art control equipment for a resistance welding machine as described above in which an inverter is used, the power loss of the diodes provided on the secondary side of the transformer is great. The power loss of the diodes reaches as much as 10-15% of the inverter output, which causes an efficiency reduction of the control equipment, and requires a water cooling system for cooling the diodes. Moreover, since a DC welding current flows, the welding electrodes are electrolyzed and their wear are severe. Therefore, as welding processes become robotized, the down time for replacing the electrodes becomes an increasing problem.
FIG. 43 is a block diagram showing another example of this type of prior art control equipment for a resistance welding machine. An AC power from an AC power source 1 is converted to a DC power by a diode bridge 2 and initially charges a capacitor 6 via a resistor 3. By timing the completion of charging capacitor 6, an electromagnetic contactor 4 is closed via an ON-delay timer 5, and resistor 3 is shorted.
The DC voltage which has been smoothed by capacitor 6, is converted to a high frequency voltage by an inverter bridge 7 and is then voltage-transformed by a transformer 8. The voltage thus transformed is converted to a DC voltage by a diode 9 and a diode 10 and is then supplied to welding electrodes 11.
In this case, a welding current is detected by a current detector 12. The setting of a welding current I is set by a current setter 13. When a starting signal ST is inputted, a conduction signal contact 14 is set ON by a conduction timer 17 and a current reference is inputted to a comparator amplifier 15.
The welding current detected by current detector 12 is inputted to comparator amplifier 15. This current is compared and amplified with the current reference to control the welding current via a PWM circuit 16 by PWM (pulse-width modulation) controlling IGBTs (insulated gate bipolar transistors) which compose inverter bridge 7.
FIG. 44 shows an example of this conduction sequence. When starting signal ST is inputted once, it is held for a 1-cycle period hold and DC current I flows during a set conduction time t1.
A time to the next conduction is longer than time t1, and generally t1/t0 is 0.1-0.05 or less. t1/t0 is generally called the "duty (%)". In the case of a resistance welding machine, time (t0-t1) is the time in which welding point determination (generally by a robot) is executed. In general, time t1 is about 1 second in most cases. Welding is performed during this time t1. For welding, pressure is applied by the electrode tip to the material to be welded, and then the welding current flows through the conducting surface thus formed. The material is heated by a generated Joule's heat Q, that is to say EQU Q=I.sup.2 .multidot.R.multidot.t1
and melts, thus forming the weld, where R is a resistance of welding portion.
Welding is performed by the Joule's heat due to a DC welding current flowing between electrodes in this way. However, as the electrodes are fixed due to the DC current, the wear of the negative (-) electrode is severe and requires frequent maintenance. Thus, this requires consumable costs and lost time.
FIG. 45 is a block diagram showing another example of a prior art control equipment for a resistance welding machine.
The AC voltage of an AC power source 201 is converted to a DC voltage by a rectifier 202. After smoothing by a capacitor 203, this DC voltage is converted to an AC voltage with a high frequency of about 1 kHz by an inverter 204 which is composed of IGBT (switching devices) 241-244. After this AC voltage is converted to a low AC voltage by a transformer 205, a DC welding current is supplied to welding electrodes 209 by converting the low AC voltage to a DC voltage by a rectifier 207. Floating inductance 208 is present in the wiring to the welding electrodes 209, and it operates effectively to smooth the DC welding current.
The magnitude of the welding current is controlled by a current reference I*. That is to say, after the primary current of transformer 205 has been detected via a current transformer 206, DC welding current is detected by simulating the DC welding current (the reflux current of rectifier 207 out of the welding current does not flow in the primary side of transformer 205) by a welding current simulator circuit 212. DC welding current I thus detected is compared with current reference I* by a current controller 213 and a current control signal a is outputted by current controller 213 so that its error is reduced. This signal a is compared by a comparator 215 with a triangular wave b outputted from a carrier generator 214, and a PWM signal c is generated based on the comparison result. Carrier generator 214 also outputs a signal d which is synchronized with the cycle of triangular wave b. A distributing circuit 216 activates either of its output signals e of f in response to signal d. By this means, PWM signal c is supplied alternately to driving circuits 212 and 222 via AND circuits 217 and 218.
A timer 220 operates when a drive signal g is inputted from a starting circuit 219, and outputs a conduction signal h only for a set time to driving circuits 221 and 222. Thus, by rendering driving circuits 221 and 222 in the operating state, switching signals j and k are outputted alternately. Thus, the group of IGBT 241, IGBT 244 and the group of IGBT 243, IGBT 242 are switched ON alternately. Therefore a high frequency AC voltage is applied from inverter 204 to the primary side of transformer 205 to control the welding current and the welding time.
With the above prior art control equipment, there is the advantage that the transformer can be miniaturized because the current control response is swift, there is little current ripple and the inverter frequency is high.
However, since a DC welding current is passed, the wear of the welding electrodes due to electrolytic action is severe. Thus, when the welding process is robotized, frequent exchange of electrodes is required. For this reason, the assembly line must be stopped temporarily, and this causes reduction of availability.
Moreover, there are also some problems, such as the occurrence of a 10% to 20% power loss in rectifier 207 which rectifies a current of several tens of thousands of amperes, and the increase of cooling water due the generation of that power loss.